In the two most important phototechnical areas of the diagnostic utilization of X-radiation, i.e. where a photosensitive film is directly illuminated by the X-rays or where the illumination occurs through an optical system, the exposure times and the film-changing time must be so specified during picture taking as to satisfy the most recent and rigorous requirements corresponding to the regulations for radiation protection and diagnosis. The exposure arrangements of the X-ray diagnosis must operate in such a way that an unexposed film, stationary during the exposure (picture taking) and fixed in the image plane, selected according to the changes of the biologic processes, is always present in the exposure area at the instant of exposure, independently of the adjustment of the equipment. The exposure times required for the utilized film are established by the X-ray generator or by the photographic camera.
Independently of the dimensions or the structure of the utilized film (be it roll film or a flat film), all the installations suitable for individual exposures comprise a film-changing device.
In the exposure arrangements most widely distributed and employed at present, the problem of changing films or of creating optimal exposure conditions is solved in various ways. In the 70 mm roll-film-changing devices or cameras the film change, with the film usually inserted into the storage cassette in a darkroom, takes place according to the film-transport methods already known with motion-picture cameras. The highest image frequency amounts in rapid-changing arrangements for roll films to six pictures per second and the exposure time to 0.11 second.
On account of the increase of the quantity of information photographed, the use of the 100 mm flat films constitutes a significant advance compared with the 70 mm roll films. The increase in the dimensions results in a 100% enlargement of the image area whereby, with maintenance of the optical parameters of the given picture-taking conditions, a larger amount of information can be extracted from the individual exposures which in terms of diagnostic analysis is of great significance.
With 100 mm flat films and with still larger dimensions, the film change with the known devices enables exposure times of only 0.04 to 0.6 second with the image frequency of 6 images per second customary in flat-film equipment. The realization of these exposure or image/dose parameters requires, in the interest of a suitable blackening of the film, a high power on the part of the X-ray generators.
The known film-changing devices suitable for 100 mm or still larger flat films solve the problem of film change as follows:
With a conventional device the unexposed films are inserted in the darkroom into a special film cassette with the individual films mutually separated inside the cassette in respective grooves. The individual films are withdrawn from the cassette inserted into the device by a lever mechanism reaching into the cassette, which operates with an advance corresponding to the spacing of the individual films, and are transferred to the film-transporting device where they are retained during exposure by a further mechanism. After the exposure the film is guided through the film-changing device into a receiving cassette.
With the described device the loading of the storage cassette in the darkroom is extremely complicated and only a maximum of 30 films at a time can be inserted into this storage cassette. The separation of the films by individual grooves, on the other hand, affords a maximum protection against static charges which can occur because of friction with the rapid relative motion of the film foils. With this device the longest still realizable exposure time at the maximum image frequency of 6 images per second amounts to 0.06 second.
With another known device, 50 or 100 films are introduced in the darkroom in a package into the storage cassette. The films are transferred from the automatically opening storage cassette, inserted into the camera, by a mechanism wherein the individual films are curved upwardly under pressure and are extracted with the aid of a gripper with relative sliding motion. During the exposure the film is retained in the image plane by a further mechanism. After the exposure the film is guided by a film-transport mechanism into the receiving cassette.
With this solution the insertion of the films into the storage cassette is relatively simple. From the mode of film transport, however, it follows that this system is very sensitive to the film thickness and the surface condition of the film is also highly significant. Thus, if the film does not have the desired parameters, major difficulties immediately result upon the separation of the individual films. Moreover, this solution offers no protection against the static charges occurring through the relative sliding of the films during film separation. It therefore can occur that the picture becomes unusable on account of a discharge. The repetition of the picture-taking operation, however, increases not only the costs but also the radiation exposure of the patient. The highest image frequency here obtainable is 2 images per second with an exposure time of 0.05 second. This low image frequency is the result of the opposite movement of the films in the film-changing device where the film separation and transport is solved in an exceedingly complicated and uneconomical manner whereby much time is lost.
The aforedescribed solutions presuppose in every instance a certain picture-taking technique whereby their utilization is tied in each case to a certain exposure arrangement and a predetermined film size.